I think it was mostly youtube and retrojunk.com

Try searching for "$channel bump"

Personally, I think Adult Swim had the best bumps, usually just some nice house music with a nice animation and some funny quotes.

I'm really interested in this but I'm having trouble googling. What is a bump? Is it referring to this? https://en.wikipedia.org/wiki/Bumper_(broadcasting)

Yup. Here's a playlist for adult swim ones. I just used yt-dlp and passed in the playlist URL and it will download all the bumps as separate files:

https://www.youtube.com/watch?v=HS8kchdwFPM&list=PL075thqiB6...

There's a torrent going around the usual places that has every [as] bump from the launch of the network to whenever they last updated the archive (a few months ago in my case.)